Protecting the Arctic

·Data from the CryoSat-2 satellite, verified using independent data, have been combined with earlier published data from NASA’s ICESat satellite to estimate trends in ice volume of the central Arctic for the period 2003–12

·Whilst the data are subject to some measurement uncertainty the observed trends confirm, and possibly, exceed the trends observed in PIOMAS simulations over the same period

·The fact that a substantial decline in Arctic sea ice volume appears to have occurred over the last 8 years does not necessarily mean that the trend will continue into the future

·CryoSat-2 data can now be used to assess, and if necessary, improve the representation of sea ice in coupled climate models to improve future predictions.

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The Centre for Polar Observation and Modelling (CPOM) is part of the National Centre for Earth Observation and conducts world-leading research into the large-scale fluctuations of the Earth’s polar ice masses. The centre provides the scientific leadership for the European Space Agency’s CryoSat-2 spacecraft that was launched in April 2010. One of the aims of the CryoSat-2 mission is the determination of ice thickness in the Arctic. When combined with data on the area of ice in the Arctic these data can be used to estimate the volume of ice in the central Arctic.

In previous evidence reference has been made to the fact that CryoSat-2 will provide data that may be used to verify model simulations of Arctic sea ice, in particular with respect to ice thickness and volume. Work has been on-going at CPOM since the launch of CryoSat-2 to tune the algorithms required to process the data to ice thickness and to validate the retrievals using independent data on ice thickness gathered from aircraft and from sub-sea moorings.

The preliminary (they are still subject to peer review) results of the validation indicate that the CryoSat-2 data agree with the independent ice thickness data to within 10 cm. The CryoSat-2 data have also been combined with satellite estimates of ice area to provide an estimate of ice volume over the central Arctic for October/November 2010 and 2011 and for February/March 2011 and 2012. These annual periods were selected to allow a comparison of ice volume estimates from the PIOMAS model (also referred to in earlier evidence) and with published estimates from NASA’s ICESat satellite for the period 2003 -2008.

The CryoSat-2 ice volume estimates for October/November 2011 and February/March 2012 agree closely (within 10%) with those from PIOMAS. Comparing CryoSat-2 data with the earlier ICESat data we observe a fall in ice volume, over the last 8 years, from ~18000 km3 to ~14000 km3 for the February/March periods and from ~14000 km3 to ~7000 km3 (~-900 km3/yr) in the October/November periods. These rates of volume decrease are higher than those from PIOMAS simulations over the same period. However, both the CryoSat-2 and ICESat data are still subject to measurement uncertainty. Even taking this uncertainty into account it would appear that these data suggest a decrease in ice volume over the period 2003–12 at least as large as that simulated by PIOMAS, and possibly higher.

Whilst these results support the substantial decline in ice volume simulated by PIOMAS over the last decade one must be cautious about using such data to extrapolate into the future. In particular it is known that ice thickness can recover from one year to the next and the observation of a past trend does not constitute proof that such a trend will continue. The next steps are first to compare these thickness data with the output of climate models (such as that from the Met Office) and second to assimilate, or initialise, climate models with CryoSat-2 data to enable a more accurate prediction of future rates of ice decline.